1. Field of the Invention
The present invention relates to a package for a semiconductor element or semiconductor elements capable of efficiently releasing heat generated within the element.
2. Description of the Prior Art
Lately, in response to the tendency toward high density circuitries, high power for high speed operation and large chips, there is a growing need for circuit elements with improved heat release. In order to meet this requirement, it has been necessary to lower thermal resistance R.sub.th(j-a) of a semiconductor package to 5.degree. C./W and preferably 2.degree. C./W. Opposed to this, is the drive towards miniaturizing devices as well as increasing pin number. It has consequently been necessary to improve the design of packages for mounting semiconductor elements.
FIG. 1 is a cross section of a conventional semiconductor package of high heat dissipation, given in Japanese Patent Laid-open No. 100758/1988.
A semiconductor element 1 formed in a multilayer interconnection structure is mounted through electrical and mechanical connections on a package substrate 2, and hermetically sealed with the aid of a metal cap 6 of kovar by seam-welding the metal cap to a seal ring 5 interposed between the substrate 2 and the metal cap 6. The package substrate 2 is made of aluminum nitride and has a plurality of input/output pins 3 on the back surface. In this example, a thermal resistance as low as 3.degree. C./W (using a heat sink 7 forcedly air-cooled at 4 m/s) was attained.
In the package structure above, since the thermal resistance of the empty space between the surface of the semiconductor and the metal cap is very high, a thermal energy generated in the semiconductor element conducts mainly by way of the package substrate 2 to heat sink 7 to be dissipated there. One problem encountered with this structure is that the path of thermal conduction between the semiconductor element 1 and the heat sink 7 is long, causing high thermal resistance of the package.
Further, as described above, since the heat energy created in the semiconductor element is not released through the metal cap, the surface of the package which is in contact with the heat sink for dissipating the heat energy should be limited only to that of the package substrate. Consequently, in order to bring enough surface of the package substrate into contact with the heat sink to attain the desired heat resistance of the package, a large heat-releasing surface area of the package substrate will be necessary, which leads to difficulty in miniaturizing the package.